Flushed pigments, that is, pigments that have been transferred as pigment particles directly to a non-aqueous phase from the aqueous phase, are widely used in printing inks and paints because they retain their fine particle structure in the flushed form, resulting in products having excellent color strength, brilliance, gloss, and transparency. Furthermore, since they do not contain hard agglomerates, they can be dispersed more readily in the selected vehicle.
In order to produce inks, paints, and the like that have these superior qualities, the flushed pigment must have certain properties, foremost among which is body length/stiffness. If the flushed pigment has too much body length, it may flow easily from the supply bin, but the finished ink may not print well.
If the flushed pigment, hereinafter referred to as "flush," has too much body stiffness, i.e., insufficient body length, it will not move out of the supply bin at normal operating pressure, and it will impart to the finished ink too much viscosity and surface tension.
Commercially flush is stored in large bins. To remove the flush, the bins are placed in apparatus equipped with a large hydraulic ram that presses the flush out of a square opening in the bottom of the bin. If the flush has insufficient flowability, it will not flow from the bin or it will flow very slowly. The measuring of the flow characteristics of a flush before it is removed from the storage bin is extremely important in order to maintain proper flow of the flush from the bin and in order not to increase production time.
In order to be controlled, the body stiffness and flowability must be measured and adjusted during manufacture. One conventional way of testing the flushed pigment is to determine the speed at which a mound of the material will spread out and to compare this with a standard, that is, a material that has been formulated to possess all of the required properties, e.g., strength, shade, tack, and flow, and that has been agreed by both the vendor and the customer to possess all of these properties within a specified range.
This procedure is not satisfactory because subtle variations in flow are not readily apparent nor can they be quantified.
Other means of measuring flowability also are not satisfactory. For example, the Ferranti Shirley instrument imparts to the flush a rotary shear; the readings obtained do not reflect or predict flowability.
The Laray Viscometer squeezes the flush between a steel rod and a cylinder; weights are put on top of the rods. The greater the weight, the faster the rod moves through the flush and the cylinder. This procedure is repeated several times to develop a slope that shows viscosity, but this subjects the flush to an input of energy in a form that is different from the energy input of a supply bin.